The investigation was narrowed down to shared IRQs. Whenever an IRQ is
set to shared, if kernel is booted with irqpoll or irqfixup, at every
interrupt received all shared interrupt handlers are triggered. This is
specially performance harming for HPET-based RTC interrupt handler,
since HPET access is known to be slow.
We proposed a simple patch [0] to address the issue, but Andy Shevchenko
(the maintainer) noticed the issue originally addressed by 079062b28fb4
("rtc: cmos: prevent kernel warning on IRQ flags mismatch") was fixed by
commit a1e23a42f1bd ("rtc: cmos: Do not assume irq 8 for rtc when there
are no legacy irqs"), originally merged in kernel 4.12.
Hence, Andy came up with a better solution for the hereby reported issue
in the form of commit: b6da197a2e96 ("rtc: cmos: Stop using shared
IRQ"), present in Focal kernel since 5.4.0-21 and to be released in
Bionic (4.15.0-94) and Eoan (5.3.0-47+). Xenial doesn't suffer from this
issue since its base kernel is 4.4 (and the offender commit was
introduced later); Xenial/HWE will get the fix organically since Bionic
has the fix.
Thanks,
Guilherme
[0] lore.kernel.org/linux-rtc/20200103140240.6507-1-gpicc...@canonical.com/T
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https://bugs.launchpad.net/bugs/1857922
Title:
hpet lost interrupts on kdump
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Eoan:
Fix Committed
Status in linux source package in Focal:
Fix Released
Bug description:
It was noticed that during the kdump kernel initialization, hpet is
getting interrupts lost, like the following message inform:
[2.816266] hpet1: lost 35 rtc interrupts
Started a bisect, the behavior is present on kernel versions v4.5 and
subsequent.
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